A typical method of operation for Laser Detection and Ranging (LADAR) system or a Laser Rangefinder (LRF) is to emit a short (typically 1-10 ns), high peak power optical pulse in a narrow beam and detect its reflected return from a target. The time delay between emission and detection provides the range to the target, and knowledge of the beam direction provides target bearing information. The transmitted beam is collimated to provide low divergence to the target so that the incident spot is sufficiently small to provide required lateral resolution.
In general, the target will reflectively scatter the incident beam into a large solid angle, so the amount of reflected power detected at the LADAR system will be proportional to the area of the receiver aperture. If the transmitted and received beams share a common aperture the system is described as monostatic. This type of system avoids misalignment and parallax problems common to LADARs and LRFs which have separate transmit and receive apertures (described as bistatic). It makes manufacturing of LIDARs and LRFs simpler since it eliminates the needed for time consuming precise alignment of the transmit and receive apertures required with bistatic systems. In monostatic operation, an optical means has to be provided for separating the transmitted and received beams before the aperture, so that the transmitted beam does not lose energy or degrade the operation of the photodetector, and the received intensity is directed primarily to the photodetector. In principle, this optical diplexing function might be as simple as a beam-splitter (which has excessive round-trip optical loss) or more complex (such as a non-reciprocal optical circulator).